In recent years there has been a growing interest in increased automation in the control of the drive train of motor vehicles, and most especially in control of the drive train of large trucks. The use of automatic transmissions in passenger automobiles and light trucks is well known, The typical automatic transmission in such a vehicle employs a fluid torque converter and hydraulically actuated gears for selecting the final drive ratio between the engine shaft and the drive wheels. This gear selection is based upon engine speed, vehicle speed and the like. It is well known that such automatic transmissions reduce the effectiveness of the transmission of power from the engine to the drive shaft, with the consummate reduction in fuel economy and power as compared with the skilled operation of a manual transmission. Such hydraulic automatic transmissions have not achieved wide spread use in large motor trucks because of the reduction in efficiency of the operation of the vehicle.
One of the reasons for the loss of efficiency when employing a hydraulic automatic transmission is loss occurring in the fluid torque converter. A typical fluid torque converter exhibits slippage and consequent loss of torque and power in all modes. It is known in the art to provide lockup torque converters that provide a direct link between the input shaft and the output shaft of the transmission above certain engine speeds. This technique provides adequate torque transfer efficiency when engaged, however, this technique provides no gain in efficiency at lower speeds.
It has been proposed to eliminate the inefficiencies inherent in a hydraulic torque converter by substitution of an automatically actuated friction clutch. This substitution introduces another problem not exhibited in the use of the hydraulic torque converters. The mechanical drive train of a motor vehicle typically exhibits considerable torsional compliance in the driveline between the transmission and the traction wheels of the vehicle. This torsional compliance may be found in the drive shaft between the transmission and the differential or the axle shaft between the differential and the driven wheels. It is often the case that independent design criteria encourages or requires this driveline to exhibit considerable torsional compliance. The existence of substantial torsional compliance in the driveline of the motor vehicle causes oscillatory response to clutch engagement. These oscillatory responses can cause considerable additional wear to the drive train components and other parts of the vehicle. In addition, these oscillatory responses can cause objectionable passenger compartment vibrations.
The oscillatory response of the driveline to clutch engagement is dependent in large degree on the manner in which the input speed of the transmission, i.e. the speed of the clutch, approaches the engine speed. A smooth approach of these speeds, such as via a decaying exponential function, imparts no torque transients on clutch lockup. If these speeds approach abruptly, then a torque transient is transmitted to the driveline resulting in an oscillatory response in the vehicle driveline. The following patents are assigned to the assignee of the present invention and represent prior developments which, in part, led to the present invention. U.S. Pat. No. 5,293,316 entitled "CLOSED LOOP LAUNCH AND CREEP CONTROL FOR AUTOMATIC CLUTCH" teaches the minimization or elimination of torsional oscillations due to compliance in the driveline during clutch engagement by controlling the clutch actuation to effect a smooth engagement. Subsequent patent applications, listed here, are improvements which make the control more robust. U.S. Pat. No. 5,275,267 entitled "CLOSED LOOP LAUNCH AND CREEP CONTROL FOR AUTOMATIC CLUTCH WITH ROBUST ALGORITHM" addresses the same problem and includes a prefilter to shape the system transient response and reduces the need for detailed particularization for individual vehicles or vehicle models. U.S. Pat. No. 5,403,249 entitled "METHOD AND APPARATUS FOR ROBUST AUTOMATIC CLUTCH CONTROL" is based on the same system and further improves robustness by overcoming the possibility of engine overload imposed by aggressive clutch engagement under certain conditions which leads to engine speed droop and even clutch dumping to avoid stalling the engine. The system as disclosed in the above specifications includes a slip integrator or actually two integrators in series which have the potential of being too sensitive to inner loop variations, leading to difficulty of control under some circumstances. Better control and increased robustness is provided by the improvement disclosed in U.S. Pat. No. 5,439,428 entitled "METHOD AND APPARATUS FOR ROBUST AUTOMATIC CLUTCH CONTROL WITH PID REGULATION."
The logic described in U.S. Pat. No. 5,378,211 entitled "CLUTCH MODE CONTROL LOGIC" determines whether the control is operating in a launch, creep, lockup or other mode. Both launch and creep require clutch slippage to control the torque transmission. In all the early developments represented by the above patents the systems have controlled clutch slippage in two distinctly separate modes and the change between creep and launch is evident to the operator.
This invention improves on the previous ones by providing a single slip mode to be used instead of the launch and creep modes. This is accomplished while increasing creep smoothness and responsiveness and low slippage of the clutch for low heat dissipation and long wear. This invention is based on that previous work and adds additional robustness. The robustness permits the mass manufacture of transmissions applicable to a wide range of heavy duty trucks without individual tuning for a given truck type or load range.